In many PEM fuel cell systems, a fuel such as methane or a similar hydrocarbon fuel is converted into a hydrogen-rich stream for the anode side of the fuel cell. In many systems, humidified natural gas (methane) and air are chemically converted to a hydrogen-rich stream known as reformate by a fuel processing subsystem of the fuel cell system. This conversion takes place in a reformer where the hydrogen is catalytically released from the hydrocarbon fuel. A common type of reformer is an Auto-thermal Reactor (ATR), which uses air and steam as oxidizing reactants. As the hydrogen is liberated, a substantial amount of carbon monoxide (CO) is created which must be reduced to a low level (typically less than 10 ppm) to prevent poisoning of the PEM membrane.
The catalytic reforming process consists of an oxygenolysis reaction with an associated water-gas shift [CH4+H2O→CO+3 H2, CO+H2O→CO2+H2] and a partial oxidation reaction [CH4+½O2→CO+2 H2]. While the water-gas shift reaction removes some of the CO from the reformate flow stream, the overall reformate stream will always contain some level of CO, the amount being dependent upon the temperature at which the reforming process occurs. After the initial reactions, the CO level of the reformate flow is well above the acceptable level for the PEM fuel cell. To reduce the CO concentration to within acceptable levels, several catalytic reactions will generally be used in the fuel processing subsystem to remove CO in the reformate flow. Typical reactions for reduction of CO in the reformate flow include the aforementioned water-gas shift, as well as a selective oxidation reaction over a precious metal catalyst (with a small amount of air added to the reformate stream to provide oxygen). Generally, several stages of CO cleanup are required to obtain a reformate stream with an acceptable CO level. Each of the stages of CO cleanup requires the reformate temperature be reduced to precise temperature ranges so that the desired catalytic reactions will occur and the loading amount of precious metal catalyst can be minimized.
For CO removal using selective oxidation, a small amount of air is added to the reformate flow to provide oxygen as required by the desired reaction. Additionally, the reformate flow is passed over a precious metal catalyst that is optimized to favor CO removal at specific temperatures. The desired reaction during a selective oxidation process is [2 CO+O2→2 CO2]. However, there are other competing reactions that are detrimental to the removal of CO from the reformate stream. Specifically, the other competing reactions are a hydrogen oxidation [H2+½O2→H2O] which converts desired hydrogen gas into water, a reverse water-gas shift [CO2+H2→CO+H2O] which creates additional harmful CO as well as depleting the amount of hydrogen gas, and a methanation [CO2+4 H2→CH4+2 H2O] which also depletes the amount of hydrogen gas in the reformate stream. FIG. 1 shows the reaction selectivity versus temperature for reverse water-gas shift reactions, CO oxidation reactions, and methanation reactions. While the catalyst and initial temperature are chosen to favor the CO oxidation over the reverse water-gas shift and methanation, temperature fluctuations cause the competing reactions to hinder CO removal performance.
In some reactors the desired temperature range for optimal CO oxidation is around 170° C. However, the CO oxidation reaction, the reverse water-gas shift reaction, as well as the methanation reaction are all exothermic, releasing heat as each respective reaction progresses. Therefore, the temperature of the reformate stream increases as much as 100° C. as it passes through a selective oxidation reactor even if the desired selective oxidation reaction initially dominates. As the temperature increases, the reaction selectivity for CO oxidation decreases with respect to the competing reactions, thereby decreasing overall CO removal efficiency.
In this regard, others have chosen to split selective oxidation reactions over multiple units with an intermediate heat exchanger to remove heat to bring the temperature of the reformate stream down to the desired temperature range, thereby bringing the reaction selectivity for CO oxidation towards an optimal level. However, such designs have inherent problems such as complex control required for multiple units and the extra cost and space required for the additional units. These problems are especially troublesome for residential applications where ease of operation and overall space considerations are essential to overall system design.